1. Field of the Invention
This invention relates to wireless communications and, more particularly, to the design of more stable low-power oscillators.
2. Description of the Related Art
Bluetooth is a wireless protocol for exchanging data over short distances (using short length radio waves) from fixed and mobile devices. Bluetooth is intended for low power applications, and is often used in devices such as faxes, mobile phones, telephones, laptops, personal computers, printers, Global Positioning System (GPS) receivers, digital cameras and video game consoles. Bluetooth uses a radio technology called frequency-hopping spread spectrum, which partitions the data being sent and transmits chunks of the data on up to 79 bands of 1 MHz width in the range 2402-2480 MHz.
A Bluetooth wireless link is formed in the context of a piconet. A piconet comprises two or more devices that occupy the same physical channel (which means that they are synchronized to a common clock and hopping sequence). The common (piconet) clock is identical to the Bluetooth clock of one of the devices in the piconet, known as the master of the piconet, and the hopping sequence is derived from the master's clock and the master's Bluetooth device address. All other synchronized devices are referred to as slaves in the piconet.
Bluetooth is a packet-based protocol with a master-slave structure. One master may communicate with up to 7 slaves in a piconet; all devices share the master's clock. Packet exchange is based on the basic clock, defined by the master, which ticks at 312.5 μs intervals. Two clock ticks make up a slot of 625 μs; two slots make up a slot pair of 1250 μs. In the simple case of single-slot packets the master transmits in even slots and receives in odd slots; the slave, conversely, receives in even slots and transmits in odd slots. Packets may be 1, 3 or 5 slots long but in all cases the master transmit will begin in even slots and the slave transmit in odd slots.
The Bluetooth specification includes a low power mode referred to as sniff mode, which more generally may be referred to as a low power sleep mode, or simply sleep mode for brevity. In sniff mode, devices that are not actively communicating can enter a low power (sleep) state, while periodically sending “keep alive” messages or transmissions to each other. In other words, in sniff mode the transmitter and receiver devices which have established a communication link periodically communicate with each other to maintain the link. For example, where a user is using a Bluetooth keyboard or mouse, and has not provided input for a certain period of time, the keyboard or mouse will enter the low power sniff mode, and the Bluetooth master device (host computer) will periodically communicate with the slave device (the keyboard or mouse) to maintain the link. Sniff mode provides the greatest benefit to battery operated human interface devices, and provides increased battery life for these devices.
The Bluetooth specification requires that a Bluetooth device maintain a 3.2 kHz Bluetooth clock, even during sleep. During sleep, Bluetooth requires that the clock be maintained to within 250 ppm+/−10 μs. Where a device includes an internal low power oscillator (LPO), the internal LPO circuitry may occasionally create a clock that drifts more than 250 ppm. This drift may be due to noise, a change in temperature, supply voltage variations, or a combination of the above.
In cases where the Bluetooth device clock drifts by more than 250 ppm, two devices may have difficulty maintaining the communication link during sniff mode. This is because, due to the difference in clocks of the master and slave devices, the master may transmit a sniff message while the slave device is asleep. For a slave device in a sniff link, it is possible for the slave device to open up its scanning window to be able to find the master transmitter. The slave device may typically open its window by a desired amount, to allow for the 250 ppm allowable clock drift error range on both sides of the link.
However, even in the case where the slave device increases its scanning window, the master device may still transmit a sniff communication when the slave device is in sleep mode. For example, the master cannot assume that the slave will open up its receive window for more than +/−250 ppm, nor can it request the slave to do so. The master device needs to perform the master transmission on time and in the appropriate frequency (as determined by the clock), or the link will drop after the expiration of a link supervision timeout (during which timeout there may be a negotiated or programmable number of attempts to revive the link).
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the embodiments described herein.